This invention relates to measuring and testing corrosion processes and it particularly relates to an improved corrosion electrode for such use.
It is often desirable to determine the rates at which metals corrode within a corrodent. For example, corrosion inhibitors are added to corrosive liquids to reduce the corrosion of exposed metals. Instruments are used to measure the rates at which these metals corrode so that the effectiveness of the inhibitor can be determined. One known method for determining rate of corrosion consists of placing a metal coupon or test specimen in a position where it is exposed to the corrosive atmosphere being studied and connecting it in an electrical circuit so that the electrical resistance of the coupon can be observed. The corroding coupon is then compared with a similar coupon placed in a protected atmosphere. The theory behind this method is that the change in electrical resistance and the change in cross-sectional area of the coupon are proportional and thus can be used to determine the rate of corrosion.
U.S. Pat. Nos. 2,856,495 to Chittum, Whittier and Armstrong, and 2,864,252 to Schaschl utilize the above method. However, the accuracy of this method depends upon the theoretical assumption that the cross-sectional area of the coupon decreases uniformly as the coupon corrodes. This is not a good assumption for many metals as they will tend to form pits (i.e., small crevices) which will fill up with metal oxides and liquid. The electrical resistance across these areas could just as well be higher or the same, as well as lower, after the metal is corroded.
To get around this problem, a new method was devised as discussed in U.S. Pat. 3,661,750 to Wilson. The measurement of the rate of corrosion upon metals by this technique utilizes an instrument associated with a probe carrying metallic electrodes. The probe and electrodes are immersed in the corrodent. The electrodes in the corrodent undergo certain electro-chemical changes that are related to the corrosion of the specimen metal of which the test electrode is formed. The rate of corrosion can be correlated with the electro-chemical effects upon the metallic test electrode contacted by the corrodent. The corrosion of metallic materials by a corrodent causes the production of electrical energy by electro-chemical action. For example, two metallic electodes immersed in a corrodent develop a potential difference as a result of half-cell effects. The potential in a freely corroding test electrode (no external current application) in a dynamic system where the corrodent products are either diffusing or dissolving, eventually reaches a certain steady-state potential differential relative to a reference electrode. This potential difference may be termed the freely corroding potential of the metallic test electrode forming the half-cell subjected to the corroding environment.
An improvement in the above system has been the use of a third electrode. With the three electrode system, the reference electrode provides a measure of the freely corroding potential developed in the electrolyte. At periodic intervals, a separate auxiliary electrode is activated by introducing a potential 10 mv higher than that measured between the test and reference electrodes. Under these conditions it has been found that the current flowing between the auxiliary and test electrodes is linearly correlatable to the corrosion rate of the electrode material in the electrolyte. This is due to the fact that (1) the small impressed voltage across the electrodes for short periods of time does not produce permanent polarization of the electrode surfaces, and (2) the major resistance in the circuit is the resistance of the film formed at the interface between the electrode and the electrolyte. Any change in the composition of the electrolyte or its corrosivity, vis-a-vis, the material of the electrode is instantaneously reflected in changes in the resistance of this film which is detected directly in a change in the current flow in the circuit. The theory on which these facts are based is discussed in detail in M. Stern and A. L. Geary, Journal of the Electrochemical Society, 104, 56 (1957).
In the past, the probes carrying metallic electrodes as described above have been placed at the inside wall perpendicular to the flow or within the flow and parallel to it. This kind of positioning causes certain inaccuracies in the corrosion rate measurement. In the case of probes placed perpendicular to the flow, inaccuracies are produced by: the turbulence caused by the probe and the electrodes protruding from it, shielding of the electrodes from the flow of the electrolyte by the adjacent electrodes, the uncertain longitudinal velocity profile across the electrodes, and the unknown horizontal velocity profile across the cylindrical surfaces of the electrodes. In the case of probes placed parallel to the flow, inaccuracies are caused by: deposits of oxides, etc., at the base of the electrodes between the holder and the electrodes, formation of air pockets at the base of the electrodes when the oxygen concentration in the flowing stream is large, and the uneven nature of the film thickness due to turbulence at the edges of the electrodes. Most of these problems can be solved by placing a cigar shaped probe parallel to the flow. The electrodes on this probe are bands wrapped around the probe which do not cause the turbulence problem that the protruding electrodes do. However, these electrodes cannot be removed for individual weighing or examination without total destruction of the probe holder.
The flush mounted electrodes of the present invention eliminate all of the above problems. Thus, accurate measurements of the corrosion rate can be made using an easily removable and relatively inexpensive electrode.